1. Field of the Invention
The present invention relates to technology for generating and sharing a common key that is common to nodes (or separate pieces of equipment) having a radio communication function to carry out radio communication based on common key cryptography, without a third party intercepting the communication.
2. Description of the Related Art
Heretofore, as a method for generating a common key, a method involves: making the same physical change in two nodes that desire cipher communication; producing shared information common to the nodes based on the physical change; and generating a common key based on the shared information. For example, Japanese Patent No. 3707660 (an eighth embodiment) discloses the following method for generating a common key. In this method, each of the two nodes is provided with a pushbutton, respectively, and information common to the nodes (e.g., the number of button presses and the timing of button press (i.e., the time interval between button presses)) is obtained by a user pressing down the pushbuttons at the nodes in the same pattern of operation. Then, a common key is generated based on this common information. According to this method, on the occasion of cipher communication between nodes, the nodes, even if not holding a common key in advance, can generate a common key and use the common key for the cipher communication.
However, the method disclosed in Japanese Patent No. 3707660 can possibly produce a situation where, even if a user can operate the pushbuttons properly at predetermined times at each of the nodes, individual pieces of information detected at each of the nodes may be different because an error occurs in sensors of the nodes that detect this operation during a sampling process. In this situation, the same common key may not be generated at each node. Although it is rare for such a situation to arise, the same common key is not always generated at each node.
Specifically, clocks of the nodes measure lengths of time for “on” and “off” conditions of the pushbuttons provided in the nodes, and the common key is generated based on the lengths of time. However, if some deviation occurs between each of the measurement clocks of the nodes, the lengths of time measured by the clocks will differ from each other. In this case, it is likely that the same common key could not be generated.
More specifically, for instance in FIG. 31 of Japanese Patent No. 3707660, if a communication device (a node) A has a timing pattern that would be “49” and a communication device (a node) B has another timing pattern that would be “50”, there is a mismatch between common keys generated respectively by communication devices A and B.